Investigations of Chlorine-Induced High Temperature Corrosion of Steels

Licentiatavhandling, 2008

The high corrosivity of the fireside environment in waste and biomass fired boilers is commonly attributed to the presence of chlorine-containing compounds. Chlorine-induced high temperature corrosion is not well understood, however. In this study, the effect of chlorine-containing compounds on corrosion is investigated by laboratory exposures and by field exposures in a commercial boiler. In the laboratory exposures polished austenitic stainless steel 310S coupons were exposed to 5% O2 in N2, with 500 ppm HCl at 500 °C for up to 168 hours. After exposure, the samples were investigated by SEM/EDX, FIB, AES and XRD. The corrosive nature of HCl was confirmed, with large amounts of FeCl2 forming in the corrosion scale. FeCl2 was not confined to the scale/metal interface but was present throughout the scale and at the scale/gas interface. Severe localised attack occurred in connection to slag inclusions in the steel surface. A new mechanism for chlorine-induced corrosion of stainless steel is proposed. The mechanism involves the simultaneous reduction of O2 and dissociation of HCl at the scale surface, and grain boundary diffusion of chloride ions and iron (II) ions through the scale, forming FeCl2. In the field exposures sample rings were mounted on air cooled probes and exposed in the 75 MW boiler in Händelö, Norrköping. The fuel was a mixture of 50% household waste and 50% industrial waste. Elemental sulphur was added to the fuel to investigate the effect of SO2 /SO3 on deposit composition and superheater corrosion. The austenitic stainless steels Sanicro 28 and 304L and the low alloyed steel 15Mo3 were exposed at 500 °C for 24 - 1000 hours. After exposure, the samples were investigated by SEM/EDX and XRD. The corrosion rate was determined after 1000 hours. Adding sulphur to the fuel reduced corrosion rate by about 50% for all three materials. In the absence of sulphur addition, large amounts of chromate formed on the stainless steels. This indicates that corrosion was initiated by chromate formation, rendering the oxide non-protective and leaving it susceptible to attack by chlorine containing compounds (e.g. HCl). The low alloyed material was unable to form a protective oxide, and formed large amounts of iron chloride. SO2 in the flue gas converts alkali chlorides and alkaline earth chlorides in the deposits into sulphates, which are considerably less corrosive towards the materials.